Twenty-four RCTs with 65,987 participants were included.
All 24 trials were double-blind and placebo-controlled. Fourteen trials had sufficient data to suggest that adequate concealment had taken place; in the other trials this was unclear. The number of patients lost to follow-up was reported in 16 trials, only one of which failed to achieve over 90% follow-up.
Gastrointestinal haemorrhage occurred in 2.47% of patients taking aspirin compared with 1.42% taking placebo (OR 1.68, 95% CI: 1.51, 1.88, p<0.0001), which was statistically significant. The NNH was 106 (95% CI: 82, 140), based on an average of 28 months therapy.
At doses below 163 mg/day, gastrointestinal haemorrhage occurred in 2.30% of patients taking aspirin, compared with 1.45% taking placebo (OR 1.59, 95% CI: 1.40, 1.81), which was statistically significant. Meta-regression showed no relation between gastrointestinal haemorrhage and dose (OR 1.015, 95% CI: 0.984, 1.047, p=0.3, per 100 mg reduction of dose).
For modified release formulations of aspirin (5 trials with 4,298 participants), the OR was 1.93 (95% CI: 1.15, 3.23).
Omitting the two largest trials did not significantly change the results.
The funnel plot indicated that additional negative studies were not found.